Abstract. The detailed molecular composition of laboratory generated limonene
ozonolysis secondary organic aerosol (SOA) was studied using
ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass
spectrometry. Approximately 1200 molecular formulas were identified in the
SOA over the mass range of 140 to 850 Da. Four characteristic groups of high
relative abundance species were observed; they indicate an array of
accretion products that retain a large fraction of the limonene skeleton.
The identified molecular formulas of each of the groups are related to one
another by CH2, O and CH2O homologous series. The CH2 and O
homologous series of the low molecular weight (MW) SOA (m/z < 300) are explained with
a combination of functionalization and fragmentation of radical
intermediates and reactive uptake of gas-phase carbonyls. They include
isomerization and elimination reactions of Criegee radicals, reactions
between alkyl peroxy radicals, and scission of alkoxy radicals resulting
from the Criegee radicals. The presence of compounds with 10–15 carbon atoms
in the first group (e.g. C11H18O6) provides evidence for SOA
formation by the reactive uptake of gas-phase carbonyls during limonene
ozonolysis. The high MW compounds (m/z > 300) were found to constitute a significant
number fraction of the identified SOA components. The formation of high MW
compounds was evaluated by molecular formula trends, fragmentation analysis
of select high MW compounds and a comprehensive reaction matrix including
the identified low MW SOA, hydroperoxides and Criegee radicals as building
blocks. Although the formation of high MW SOA may occur via a variety of
radical and non-radical reaction channels, the combined approach indicates a
greater importance of the non-condensation reactions over aldol and ester
condensation reaction channels. Among these hemi-acetal reactions appear to
be most dominant followed by hydroperoxide and Criegee reaction channels.